1. Field of the Invention
The invention relates to aqueous two-component coating compositions for automotive and industrial coatings based on hydroxy-functional acrylate copolymers and polyisocyanate cross-linking agents, which are preferably used in vehicle refinishing.
2. Description of Related Art
For ecological reasons water based paints are increasingly used in automotive and industrial coatings. The aim of this is to keep the proportion of organic co-solvents in the water based paints as small as possible. The water-dilutable binders, for example acrylate copolymers, used in water paints are usually manufactured in organic solvents, the solvents being at least partly distilled off at a later stage.
In U.S. Pat. No. 5,773,513 aqueous coating compositions based on water-dilutable acrylate resins are described, the acrylate resins being manufactured in several stages by polymerization in organic solvents. In this procedure a mono-epoxy ester and organic solvent are introduced, in a first stage unsaturated acid-functional monomers, unsaturated hydroxy-functional monomers and vinyl aromatic monomers are added and polymerized and in a second stage the remaining unsaturated acid-functional monomers and, possibly, further unsaturated monomers are added and polymerised. After neutralization and transfer to the aqueous phase surplus solvent is distilled off. Besides the fact that the distilling off represents an additional step in the binder manufacture, there is also the disadvantage of incompatibility of the acrylate resins with hydrophobic polyisocyanates.
EP 619 329 (U.S. Pat. No. 5,539,022) describes water-dilutable dispersions of acrylate copolymers with hydroxyl values of 40 to 200 mg KOH/g, which were manufactured by reaction of 5 to 40% by weight of a glycidyl ester derived from a branched aliphatic saturated monocarboxylic acid in alpha position, 0 to 30% by weight of a diester derived from an olefinic unsaturated dicarboxylic acid, 0 to 70% by weight of a vinyl aromatic hydrocarbon, 0 to 60% by weight of an alkyl ester derived from an olefinic unsaturated carboxylic acid, 2 to 40% by weight of an olefinic unsaturated carboxylic acid and 5 to 40% by weight of a hydroxy alkyl ester derived from an olefinic unsaturated carboxylic acid. From the water-dilutable dispersions of acrylate resins manufactured in this way, water-dilutable clear coat baking enamels or water-dilutable solid-color topcoat baking enamels are produced by admixing an amino resin as crosslinking agent.
Thus, there is a need for water-dilutable two-component coating compositions based on hydroxy-functional polymers and polyisocyanates with free isocyanate groups, which contain acrylate copolymers being able to be manufactured without difficulty and without compatibility problems in the substance or in the presence of only very small quantities of organic solvents. The hydroxy-functional (meth)acrylcopolymers should also be highly compatible with hydrophobic polyisocyanates, i.e. polyisocyanates that are not especially hydrophilically modified and be easily miscible with them. The miscibility should be guaranteed without having to add larger amounts of organic co-solvents and/or high shear 0rates. Furthermore, the coatings should display a high solids content at spray viscosity.